This invention relates to a process for the production of polyurethane foams which are resistant to weathering and virtually non-yellowing. The process according to the present invention is distinguished by the use of novel catalysts for the polyaddition reaction.
Foams based on polyisocyanates, e.g. polyurethane foams having a dense skin and cellular core such as are obtained by the method of foaming in molds (German Auslegeschriften Nos. 1,196,864 and 1,694,138 and French Pat. No. 1,559,325) are eminently suitable for the mass production of lightweight building constructions, e.g. for the furniture, motor vehicle or construction industry, or for the production of elastic molded articles such as cushions, shock absorbers or shoe soles. To produce the molded polyurethane articles, the foamable reaction mixture consisting of polyisocyanates, compounds carrying at least two isocyanate reactive hydrogen atoms and additives is introduced into a closed heatable mold in which it foams up and solidifies in a highly compressed state. The material exactly fills the mold and reproduces the internal surface of the mold.
The rigid and semi-rigid polyurethane foams hitherto brought on the market are mainly polyaddition products of aromatic polyisocyanates with polyether or polyester polyols. Although these products have superior mechanical properties which make them suitable for many of the applications mentioned above, they have the disadvantage of turning yellow as a result of weathering and, in addition, form a rough surface due to chemical degradation. If these known products are to be exposed to severe conditions in use, they must be covered with a protective layer of lacquer.
It is known that polyurethanes which are based on aliphatic or alicyclic isocyanates do not show this yellowing. Attempts, however, to produce molded polyurethane articles on the basis of aliphatic and alicyclic isocyanates have hitherto failed due to the problem of catalysis. It is not possible in the production of molded foams to obtain the typical structure of a compact skin and cellular core with the density gradually decreasing from the wall to the center of the foam by carrying out a subsequent heat treatment. It is therefore necessary to control the reaction by means of a suitable catalyst so that it begins after a short dwell time of the liquid mixture of starting components and is completed within a few seconds during which the material foams up and solidifies. Only in this way is it possible to ensure that the desired typical density gradient in the molded article is obtained as a result of the condensation of blowing agent on the walls of the mold and evaporation of the blowing agent in the interior of the mold.
It has now been found that excellent rigid and semi-rigid molded foams which undergo virtually no yellowing as a result of weathering and only very slight chemical degradation due to weathering can be produced by using aliphatic and/or alicyclic isocyanates as the polyisocyanate component together with amine-free catalyst combinations of alkali metal and/or alkaline earth metal hydroxide and certain organic metal catalysts.